1. Field of the Invention
The present invention relates to an ink jet printing apparatus and an ink jet printing method for performing printing by using a printing head that can eject black ink and at least one color ink.
2. Description of the Related Art
Conventionally, since an ink jet printing apparatus can print various types of printing media at a high density and at a high speed, ink jet printing apparatuses are widely employed as output apparatuses, such as printers or portable printers, for various apparatuses, and are sold commercially.
A general ink jet printing apparatus of a serial scan type comprises: a carriage mounting a printing head, which serves as printing means, and an ink tank; conveying means, for conveying a printing medium; and control means, for controlling these means. The ink jet printing apparatus repeats a serial scanning operation, in which a printing head that can eject ink droplets through a plurality of ejecting ports is moved in a direction (a main scanning direction) perpendicular to a direction (a sub-scanning direction) in which the printing medium is conveyed, and an operation in which the printing medium is conveyed a distance equivalent to a printing width. By this printing method, based on a printing signal, ink is ejected onto a printing medium to perform printing, and this printing method is frequently employed because the running costs are low and less noise is produced. Recently, furthermore, many products that employ a plurality of color inks, and that can be applied for color image printing, have been developed and put to practical use.
Since a color ink jet printing apparatus frequently employs black ink for printing characters, the production of a sharp, clear image and the acquisition of a high printing density are required. Thus, a technique is well known whereby the permeability of black ink relative to a printing medium is reduced, to prevent color material included in black ink from permeating printing medium.
As for color inks other than black, when two differently color inks are ejected onto a printing medium so they are adjacent to each other, the two colors may mix at the common boundary between them, and the quality of a printed color image may be deteriorated (boundary bleeding). To prevent this boundary bleeding, a technique has been is disclosed whereby the permeability of color inks relative to the printing medium is increased to prevent the inks from mixing on the surface of the printing medium (e.g., Japanese Patent Application Laid-open No. 55-65269(1980)).
However, when the black ink and the color inks described above are employed, the following two problems have arisen.
First, while the fixing time for color ink is short because of its high permeability, the drying and fixing time for black ink is extended because of its low permeability. Therefore, when continuous printing is performed by using a plurality of pages as the printing media, a problem occurs. That is, when printed pages are discharged and overlaid sequentially while black ink attached to the pages has not dried completely, the printing face of a page will rub against the reverse face of the following page and be smudged. This smudging occurring on the printing and the reverse face of the printing medium is called “smearing”. This problem becomes ever more noticeable as the printing speed is increased.
Second, since the permeability of black ink is low, boundary bleeding may occur on a printing medium at a common boundary between black ink and color ink. And considerable deterioration of the printing quality of a color image occurs.
To resolve the two problems, the following countermeasures are conventionally taken.
A first countermeasure is a method that uses an ink fixing means such as a thermal fixing device. According to this method, ink can be rapidly fixed to a printing medium, and the occurrence of smearing and boundary bleeding can be prevented.
A second countermeasure is a method for delaying the post-printing discharge of a printing medium. According to this method, during a period following the printing of an image, on a first printing sheet, that continues until the ink has completely dried, the printing of a second sheet is temporarily delayed, or after an image has been printed on the second printing sheet, the discharge of this printing sheet is temporarily delayed until the ink on the first printing sheet has dried. Using this method, the occurrence of smearing can be prevented.
A third countermeasure is a method for overlaying an area wherein black ink is to be attached with highly permeable color ink. Since black ink is to be provided for the sheet face whereon the color ink is attached, black ink can be easily fixed to the sheet, and the occurrence of smearing can be prevented. Furthermore, since a set of black ink and color inks that react with and coagulate to each other is employed, the occurrence of boundary bleeding can be prevented.
However, the above described countermeasures have the following shortcomings.
For the first countermeasure, a shortcoming is that an increase in the size and the cost of a printing apparatus can not be avoided because a fixing means is required. Further, for a serial printer, since the printing medium conveying operation is intermittently performed, the distances traveled by printing medium conveyed to the fixing device may be uneven.
For the second countermeasure, a shortcoming is that throughput is low because the discharge of printing media is delayed.
For the third countermeasure, a shortcoming is that since color inks are overlaid on a printing medium, the sharpness of a black image and the printing quality of black characters may be reduced. Further, when the amount of color ink required to prevent smearing differs from the amount of color ink required to prevent boundary bleeding, it is difficult to prevent the occurrence both of smearing and of boundary bleeding.